Ultrasonic probe, ultrasonic diagnostic apparatus, and method for manufacturing backing material

ABSTRACT

An ultrasonic probe includes: a piezoelectric element; and a backing material including a matrix resin and thermally conductive particles, arranged on one direction side with respect to the piezoelectric element, wherein a ratio of thermal conductivity of the backing material in a thickness direction to the thermal conductivity of the backing material in a horizontal direction is 3 or more.

The entire disclosure of Japanese patent Application No. 2019-190145,filed on Oct. 17, 2019, is incorporated herein by reference in itsentirety.

BACKGROUND Technological Field

The present invention relates to an ultrasonic probe, an ultrasonicdiagnostic apparatus, and a method for manufacturing a backing material.

Description of the Related Art

By simple operation of applying an ultrasonic probe connected to anultrasonic diagnostic apparatus or communicable with the ultrasonicdiagnostic apparatus to a body surface of a subject including humanbeing and other animals or inserting the ultrasonic probe into the body,the ultrasonic diagnostic apparatus can obtain the shape, movement, orthe like of a tissue as an ultrasonic diagnostic image. The ultrasonicdiagnostic apparatus can repeatedly perform an examinationadvantageously because of high safety.

The ultrasonic probe incorporates, for example, a piezoelectric elementthat transmits and receives an ultrasonic wave. The piezoelectricelement receives an electric signal (transmission signal) transmittedfrom the ultrasonic diagnostic apparatus, converts the receivedtransmission signal into an ultrasonic signal to transmit the ultrasonicsignal, receives an ultrasonic wave reflected in a living body toconvert the ultrasonic wave into an electric signal (reception signal),and transmits the reception signal converted into the electric signal tothe ultrasonic diagnostic apparatus.

In addition, the ultrasonic probe includes a backing material on theside opposite to a surface of the piezoelectric element facing a subject(note that hereinafter, regarding a member forming the ultrasonic probe,a surface facing an ultrasonic irradiation direction (surface facing asubject) is also referred to as “front surface”, and a surface facingthe direction opposite to the ultrasonic irradiation direction (surfaceopposite to the surface facing the subject) is also referred to as “backsurface”). The backing material attenuates (includingabsorption/scattering) the ultrasonic wave transmitted from thepiezoelectric element to a back surface side to suppress generation ofnoise (artifact) or the like due to reflection of the ultrasonic wavetransmitted to the back surface side on an end surface of the backingmaterial. In addition, the backing material dissipates heat from thepiezoelectric element to the back surface side to suppress overheat orthe like of an acoustic lens in contact with the subject due to heatgenerated in the piezoelectric element.

Therefore, various backing materials each having a higher thermalconductivity have been studied.

For example, JP 2017-527375 A discloses an ultrasonic probe including: atransducer assembly that can be operated so as to propagate ultrasonicenergy; and a cooling system including a heat transfer device disposedso as to transfer heat generated by the transducer assembly. Byinclusion of a graphene-based material or graphene to which anothercomponent such as a resin has been added in order to obtain a compositematerial in the heat transfer device, the ultrasonic probe has afavorable thermal conductivity.

However, as a result of studies of the present inventor, when it wastried to increase the thermal conductivity of a backing material bygraphene described in JP 2017-527375 A, it was necessary to add a largeamount of graphene to the backing material, it was difficult to makeboth mixing and moldability favorable, and desired thermally conductiveperformance of the backing material could not be obtained. In addition,when the backing material contains a large amount of graphene, anultrasonic wave transmitted to a back surface side is not sufficientlyattenuated disadvantageously.

SUMMARY

The present invention has been achieved in view of the above points, andan object of the present invention is to provide an ultrasonic probeincluding a backing material having high thermally conductiveperformance and favorable acoustic characteristics, an ultrasonicdiagnostic apparatus including the backing material, and a method forpreparing the backing material.

To achieve the abovementioned object, according to an aspect of thepresent invention, an ultrasonic probe reflecting one aspect of thepresent invention comprises: a piezoelectric element; and a backingmaterial including a matrix resin and thermally conductive particles,arranged on one direction side with respect to the piezoelectricelement, wherein a ratio of thermal conductivity of the backing materialin a thickness direction to the thermal conductivity of the backingmaterial in a horizontal direction is 3 or more.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages and features provided by one or more embodiments of theinvention will become more fully understood from the detaileddescription given hereinbelow and the appended drawings which are givenby way of illustration only, and thus are not intended as a definitionof the limits of the present invention:

FIG. 1 is a cross-sectional view illustrating an example of an entirestructure of an ultrasonic probe according to an embodiment of thepresent invention;

FIG. 2A is a cross-sectional view of a backing material of theultrasonic probe according to the embodiment of the present invention;

FIG. 2B is a cross-sectional view of a backing material of an ultrasonicprobe of Comparative Example;

FIG. 3 is a schematic view illustrating an example of an ultrasonicdiagnostic apparatus including the ultrasonic probe according to theembodiment of the present invention; and

FIGS. 4A and 4B are graphs illustrating a heat dissipation effect of theultrasonic probe according to the embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, one or more embodiments of the present invention will bedescribed with reference to the drawings. However, the scope of theinvention is not limited to the disclosed embodiments.

1. Ultrasonic Probe

FIG. 1 is a cross-sectional view illustrating an example of an entirestructure of an ultrasonic probe 100 according to an embodiment of thepresent invention.

As illustrated in FIG. 1, the ultrasonic probe 100 includes: apiezoelectric element 110; a ground electrode 120 disposed on a frontsurface side and a signal electrode 130 and a signal electric terminal140 disposed on a back surface side for applying a voltage to thepiezoelectric element 110; an acoustic matching layer 150 and anacoustic lens 160 disposed on the front surface side in this order fromthe piezoelectric element 110; and a backing material 170 disposed onthe back surface side from the signal electric terminal 140.

1-1. Piezoelectric Element

The piezoelectric element 110 is formed by arranging a plurality ofpiezoelectric bodies (not illustrated) that transmits an ultrasonic waveby application of a voltage one-dimensionally in an X direction inFIG. 1. The thickness of the piezoelectric element 110 can be, forexample, 0.05 mm or more and 0.4 mm or less. Each of the piezoelectricbodies is formed by a piezoelectric ceramic such as lead zirconatetitanate (PZT), a piezoelectric single crystal such as lead magnesiumniobate/lead titanate solid solution (PMN-PT) or lead zirconateniobate/lead titanate solid solution (PZN-PT), a composite piezoelectricbody obtained by combining these materials and a polymer material, orthe like. Note that the piezoelectric element 110 usually has amagnitude of an acoustic impedance of 10 to 30 MRayls.

1-2. Ground Electrode, Signal Electrode, and Signal Electric Terminal

The ground electrode 120 is an electrode disposed on a front surface ofthe piezoelectric element 110, and the signal electrode 130 is anelectrode disposed on a back surface of the piezoelectric element 110.The ground electrode 120 and the signal electrode 130 can be formed byvapor-depositing or sputtering gold, silver, and the like, bakingsilver, or attaching a conductor such as copper to an insulatingsubstrate and patterning the conductor. The signal electric terminal 140is disposed in contact with a back surface side of the signal electrode130, and connects the signal electrode 130 to an external power sourceor the like disposed in a main body 11 of the ultrasonic diagnosticapparatus 10. In the present embodiment, the signal electrode 130 is aflexible printed circuit (FPC) formed by attaching a conductor such ascopper to an insulating substrate and patterning the conductor.

1-3. Acoustic Matching Layer

The acoustic matching layer 150 is a layer for matching acousticcharacteristics between the piezoelectric element 110 and the acousticlens 160, and is formed by a material having an acoustic impedancesubstantially intermediate between that of the piezoelectric element 110and that of the acoustic lens 160. In the present embodiment, theacoustic matching layer 150 includes three layers of a first acousticmatching layer 150 a, a second acoustic matching layer 150 b, and athird acoustic matching layer 150 c.

Here, the first acoustic matching layer 150 a is formed by a materialsuch as graphite impregnated with silicon, quartz, free-machiningceramics, or metal, graphite filled with metal particles, or an epoxyresin filled with a filler such as metal or an oxide, having an acousticimpedance of 8 MRayls or more and 20 MRayls or less. The second acousticmatching layer 150 b is formed by graphite or an epoxy resin filled witha filler such as metal or an oxide, having an acoustic impedance of 3MRayls or more and 8 MRayls or less. The third acoustic matching layer150 c is formed by a plastic material mixed with a rubber material, aresin filled with a silicone rubber, or the like, having an acousticimpedance of 1.9 MRayls or more and 2.3 MRayls or less.

By making the acoustic matching layer 150 multilayered in this way, theband of the ultrasonic probe can be widened. Note that when the acousticmatching layer 150 is multilayered, the acoustic impedance of each layeris more preferably set such that the acoustic impedance of the acousticmatching layer 150 approaches the acoustic impedance of the acousticlens 160 stepwise or continuously as the acoustic matching layer 150 iscloser to the acoustic lens 160. In addition, the layers of themultilayered acoustic matching layer 150 may be bonded to each otherwith an adhesive usually used in the present technical field, such as anepoxy-based adhesive.

Note that the material of the acoustic matching layer 150 is not limitedto the above materials, and may be aluminum, an aluminum alloy (forexample, an AL-Mg alloy), a magnesium alloy, macor glass, glass, fusedquartz, copper graphite, polyethylene (PE), polypropylene (PP),polycarbonate (PC), an ABC resin, an ABS resin, an AAS resin, an AESresin, nylon (PA6 or PA6-6), polyphenylene oxide (PPO), polyphenylenesulfide (PPS: PPS containing glass fiber is also available),polyphenylene ether (PPE), polyether ether ketone (PEEK), polyamideimide (PAI), polyethylene terephthalate (PETP), an epoxy resin, aurethane resin, or the like. A material obtained by adding zinc oxide,titanium oxide, silica, alumina, red iron oxide, ferrite, tungstenoxide, yttrium oxide, barium sulfate, tungsten, molybdenum, or the likeas a filler to a thermosetting resin such as an epoxy resin and moldingthe resulting mixture is preferable.

1-4. Acoustic Lens

The acoustic lens 160 is formed by a polymer material or the like havingan acoustic impedance close to that of a living body and having a soundvelocity different from that of the living body, and focuses ultrasonicwaves transmitted from the piezoelectric element 110 using refractiondue to a difference in sound velocity between the living body and theacoustic lens 160 to improve resolution. In the present embodiment, theacoustic lens 160 is a cylindrical acoustic lens extending in a Ydirection (direction orthogonal to an arrangement direction X of thepiezoelectric bodies) in the drawing and having a convex shape in a Zdirection. The acoustic lens 160 focuses the ultrasonic waves in the Ydirection and emits the ultrasonic waves to the outside of theultrasonic probe 100.

The acoustic lens 160 may be, for example, a homopolymer such as a knownsilicone-based rubber, a butadiene-based rubber, a polyurethane rubber,or an epichlorohydrin rubber, or a copolymer rubber such as anethylene-propylene copolymer rubber obtained by copolymerizing ethyleneand propylene. Among these materials, the silicone-based rubber and thebutadiene-based rubber are preferably used.

Examples of the silicone-based rubber include a silicone rubber and afluorosilicone rubber. The silicone rubber refers to anorganopolysiloxane having a molecular skeleton formed by Si—O bonds, inwhich a plurality of organic groups is mainly bonded to the Si atoms.Usually, a main component of the organopolysiloxane is methylpolysiloxane, and 90% or more of all the organic groups are methylgroups. A substance into which a hydrogen atom, a phenyl group, a vinylgroup, an allyl group, or the like is introduced instead of a methylgroup can also be used. The silicone rubber can be obtained, forexample, by kneading a curing agent (vulcanizing agent) such as benzoylperoxide with an organopolysiloxane having a high polymerization degree,and heating and vulcanizing the kneaded product to cure the kneadedproduct. If necessary, an organic or inorganic filler such as silica ornylon powder, a vulcanization aid such as sulfur or zinc oxide, or thelike may be added.

Examples of the butadiene-based rubber include a polymer rubber obtainedby polymerizing butadiene alone and a copolymer rubber obtained bycopolymerizing butadiene as a main component with a small amount ofstyrene or acrylonitrile. The butadiene rubber is a synthetic rubberobtained by polymerizing butadiene having a conjugated double bond. Thebutadiene rubber can be obtained by 1.4-polymerizing or 1.2-polymerizingbutadiene alone having a conjugated double bond. The butadiene rubbermay be vulcanized with sulfur or the like.

Examples of a commercially available silicone rubber include: KE742U,KE752U, KE931U, KE941U, KE951U, KE961U, KE850U, KE555U, and KE575U (allof which are manufactured by Shin-Etsu Chemical Co., Ltd.); TSE221-3U,TE221-4U, TSE2233U, XE20-523-4U, TSE27-4U, TSE260-3U, and TSE-260-4U(all of which are manufactured by Momentive Performance Materials Inc.);and SH35U, SH55UA, SH831U, SE6749U, SE1120U, and SE4704U (all of whichare manufactured by Toray Dow Corning Co. Ltd.).

1-5. Backing Material

The backing material 170 is a layer that holds the piezoelectric element110, simultaneously attenuates an ultrasonic wave transmitted from thepiezoelectric element 110 to a back surface side, and releases heatgenerated from the piezoelectric element 110 to the back surface side.Examples of a base material (matrix resin) of the backing material 170include a natural rubber, a ferrite rubber, an epoxy resin, polyvinylchloride, polyvinyl butyral (PVB), an ABS resin, polyurethane (PUR),polyvinyl alcohol (PVAL), polyethylene (PE), polypropylene (PP),polyacetal (POM), polyethylene terephthalate (PETP), fluororesin (PTFE)polyethylene glycol, a polyethylene terephthalate-polyethylene glycolcopolymer, and other thermoplastic resins. Among the above basematerials (matrix resins), the epoxy resin is preferable.

Examples of the epoxy resin include a novolac type epoxy resin such as abisphenol A type, a bisphenol F type, a resole novolac type, or a phenolmodified novolac type, a polycyclic aromatic epoxy resin such as anaphthalene structure-containing type, an anthracenestructure-containing type, or a fluorene structure-containing type, ahydrogenated alicyclic epoxy resin, a liquid crystalline epoxy resin,and a powder epoxy resin.

The matrix resin contained in the backing material 170 according to thepresent embodiment is preferably made of a powder resin. The matrixresin is preferably made of powder particles having a number averageparticle size of 10 μm or more and 200 μm or less, more preferablypowder particles having a number average particle size of 10 μm or moreand 100 μm or less, still more preferably powder particles having anumber average particle size of 30 μm or more 70 μm or less. The shapeof the backing material 170 is not particularly limited as long as beingable to attenuate a transmitted ultrasonic wave.

As illustrated in FIG. 2A, thermally conductive particles included inthe backing material 170 according to the present embodiment areoriented in the thickness direction of the backing material 170. Here,the thickness direction of the backing material means a directionperpendicular to a surface facing an ultrasonic irradiation direction(surface facing a subject) with a surface facing the direction oppositeto the ultrasonic irradiation direction (surface opposite to the surfacefacing the subject) as a bottom surface. In addition, “oriented” meansthat the thermally conductive particles are aggregated and arranged inthe thickness direction of the backing material. In addition,aggregation means that a plurality of thermally conductive particlesgathers to form a stripe. Meanwhile, thermally conductive particlesincluded in a backing material of an ultrasonic probe of ComparativeExample illustrated in FIG. 2B are dispersed in the backing materialwithout being oriented.

When the resin is made of powder, the powder particles of the resin canbe arranged along an interface (surface) of the thermally conductiveparticles. Therefore, the thermally conductive particles in the backingmaterial pressurized during molding can be oriented in the thickness ofthe backing material. In addition, the backing material can have aregion where the thermally conductive particles are aggregated and aregion where the thermally conductive particles are not aggregated. As aresult, the orientation of the thermally conductive particles can beimproved. Therefore, the thermal conductivity of the backing materialcan be improved to 30 W/mk in the thickness direction and 6 W/mk in thehorizontal direction.

In addition, a ratio of the thermal conductivity of the backing material170 according to the present embodiment in the thickness direction tothe thermal conductivity thereof in the horizontal direction ispreferably 3 or more and 10 or less, and more preferably 4 or more and 5or less. If the ratio of the thermal conductivity of the backingmaterial 170 in the thickness direction to the thermal conductivitythereof in the horizontal direction is 3 or more, even when thetemperature inside the ultrasonic probe 100 rises to about 50 to 60° C.during use of the ultrasonic probe 100, heat can be quickly dissipatedfrom the direction opposite to a subject side. Note that the thermalconductivity of the backing material according to the present embodimentin each of the thickness direction and the horizontal direction can bemeasured by a laser flash method according to JIS R1611:2010.

The backing material 170 enhances the thermal conductivity by containingthe thermally conductive particles.

A ratio of the sound velocity of the backing material 170 according tothe present embodiment in the thickness direction to the sound velocitythereof in the horizontal direction is preferably 0.5 or more, and morepreferably 0.6 or more and 1.0 or less. By setting the ratio of thesound velocity of the backing material 170 in the thickness direction tothe sound velocity thereof in the horizontal direction to 0.6 or more,it is possible to suppress anisotropy and to obtain not only desiredthermally conductive performance but also desired attenuation. Inaddition, by setting the ratio of the sound velocity of the backingmaterial 170 in the thickness direction to the sound velocity thereof inthe horizontal direction to 1.0 or less, it is possible to suppress anincrease in only one function (for example, thermally conductiveperformance). Note that the sound velocity of the backing material 170in each of the thickness direction and the horizontal direction can bedetermined according to JIS Z2353:2003.

The attenuation of the backing material 170 according to the presentembodiment is preferably 9 dB/mm·MHz or more, and the acoustic impedancethereof is preferably 1.5 to 3.0 Mrayls.

1-5-1. Thermally Conductive Particles

The thermally conductive particle contain a thermally conductivematerial. The thermally conductive material has a thermal conductivityof preferably 60 to 5000 w/mk, more preferably 200 to 3000 w/mk, stillmore preferably 400 to 3000 w/mk from a viewpoint of easily adjustingacoustic characteristics while further enhancing the thermallyconductive performance of the backing material 170. Examples of thethermally conductive material having a thermal conductivity within theabove range include aluminum oxide, silicon carbide, aluminum nitride,silicon nitride, beryllium oxide, boron nitride, magnesium oxide,graphene, a carbon nanotube, aluminum, gold, silver, iron, and copper.The thermally conductive particles may contain one or more kindsselected from these thermally conductive materials. In addition, thethermally conductive particles may contain a material other than theabove thermally conductive materials, like composite particles describedlater.

Here, a state in which the backing material 170 has favorable acousticcharacteristics means that the attenuation ratio of an ultrasonic waveby the backing material 170 is sufficiently high, the backing material170 has a sound velocity suitable for measurement, or the backingmaterial 170 has an acoustic impedance to such an extent that thebacking material 170 can appropriately reflect an ultrasonic wavetransmitted from the piezoelectric element 110. Note that morepreferably, the attenuation ratio of an ultrasonic wave by the backingmaterial 170 is sufficiently high, and the backing material 170 has anacoustic impedance to such an extent that the backing material 170 canappropriately reflect an ultrasonic wave transmitted from thepiezoelectric element 110.

Here, the attenuation ratio of an ultrasonic wave is preferably higherfrom a viewpoint of suppressing generation of noise (artifact). When abetter-shaped waveform is desired even if sensitivity is sacrificed tosome extent, a value of the acoustic impedance can be increased. Whenthe sensitivity is desired to be increased, the value of the acousticimpedance can be decreased. In addition, the shape of a band can also becontrolled by the value of the acoustic impedance. When the value of theacoustic impedance is low, the band is wide. When the value of theacoustic impedance is high, the band is narrow.

The thermally conductive material is preferably multilayer (ML)graphene, silicon carbide, or a carbon nanotube from a viewpoint ofadjusting the orientation state of the thermally conductive particles toeasily adjust the acoustic characteristics.

Note that the orientation state of the thermally conductive particles ispreferably adjusted such that aggregation of the thermally conductiveparticles in the backing material 170 is suppressed. As a result, it ispossible to suppress a variation in the thermal conductivity andacoustic characteristics of the backing material 170, to enhance thethermally conductive performance of the backing material 170, and toimprove the acoustic characteristics.

In addition, when the amount of the thermally conductive particlescontained in the backing material 170 is large, the durability of thebacking material 170 is likely to decrease, or the acoustic impedance ofthe backing material 170 is likely to deviate from the suitable range.

Note that the amount of the thermally conductive particles contained inthe backing material 170 is preferably smaller, while the backingmaterial 170 preferably contains the thermally conductive particles insuch an amount that overheat of the acoustic lens 160 can be suppressedfrom a viewpoint of suppressing aggregation of the thermally conductiveparticles. Specifically, by containing the thermally conductiveparticles, the backing material 170 has a thermal conductivity ofpreferably 2.0 W/mk or more, more preferably 4.0 W/mk or more, stillmore preferably 10.0 W/mk or more, particularly preferably 20.0 W/mk ormore.

However, according to finding of the present inventor, by simplyincreasing the amount of the thermally conductive particles contained inthe backing material 170, the thermally conductive particles are moreeasily aggregated, and therefore the thermal conductivity of the backingmaterial 170 does not rise as expected. In order to efficiently enhancethe thermal conductivity of the backing material 170, it is necessary toadjust the orientation state of the thermally conductive particles todisperse the thermally conductive particles more suitably. In thepresent invention, by using the matrix resin made of a powder resin, dueto an excluded volume effect of particles such as the thermallyconductive particles contained in the backing material, the thermallyconductive particles can be oriented at regular intervals even if thethermally conductive particles are not uniformly dispersed at the timeof mixing. As a result, the thermal conductivity of the backing materialcan be improved up to 30 W/mk in the thickness direction and up to 6W/mk in the horizontal direction.

In addition, the thermally conductive particles have a number averageparticle size of preferably 10 μm or more and 150 μm or less, morepreferably 10 μm or more and 100 μm or less from a viewpoint of easilyadjusting the orientation state of the thermally conductive particles.When the number average particle size of the thermally conductiveparticles is within the above range, the thermally conductive particlesare more easily dispersed, and the orientation state of the thermallyconductive particles is more easily adjusted as compared with a case ofusing particles having a smaller number average particle size. Note thathere, the number average particle size of particles is a value measuredusing a laser particle size distribution measuring device.Alternatively, here, the number average particle size of the particlescontained in the backing material 170 may be a value obtained by thinlycutting the backing material 170, imaging the cut backing material 170with a transmission electron microscope at a magnification of about1,000,000 times, and analyzing the obtained image with well-knownanalysis software.

1-5-2. Composite Particles

The thermally conductive particles may be composite particles from aviewpoint of suppressing aggregation of the thermally conductiveparticles to easily adjust the orientation state of the thermallyconductive particles. By using the composite particles, a particleconcentration in an epoxy resin can be reduced, and therefore a particlesurface is easily covered with the epoxy resin. This can suppressgeneration of bubbles and cracks during molding of the backing material170, and can suppress a change in capacity during dicing.

The composite particles are obtained by combining particles made of theabove-described thermally conductive material with a material other thanthe thermally conductive material (for example, an elastomer). Thecomposite particles may further contain a filler and the like.

The elastomer is a substance having rubber elasticity at roomtemperature. The elastomer may be a thermosetting elastomer or athermoplastic elastomer.

Examples of the thermoplastic elastomer include a polyester elastomer, apolyamide elastomer, a polyether elastomer, a polyurethane elastomer, apolyolefin elastomer, a polystyrene elastomer, a polyacrylic elastomer,a polydiene elastomer, a silicone-modified polycarbonate elastomer, anda fluorine copolymer elastomer.

Examples of the thermosetting elastomer include a flexible epoxy resin,a silicone resin, an isoprene rubber, an ethylene propylene rubber, abutadiene rubber, a chloroprene rubber, and a natural rubber.

Since the ultrasonic probe is sterilized in a high temperature gasenvironment or the like, the elastomer is preferably a thermosettingelastomer that is unlikely to be deformed or flown by a change intemperature, and is more preferably a silicone resin.

The composite particles can be manufactured by pulverizing a mixture ofmaterials of the composite particles with a pulverizer. At this time,the elastomer is preferably a material having a short elongation atbreak and a lower hardness from a viewpoint of easily pulverizing themixture to facilitate manufacture of the composite particles, and from aviewpoint of suppressing generation of bubbles and cracks due to damageof the composite particles during molding of the backing material 170.

Note that pulverization of the composite particles may be pulverizationat room temperature or freeze pulverization. An impact type pulverizersuch as a turbo mill, a pin mill, a hammer mill, or a Linrex mill can beused for pulverizing the composite particles. In the freezepulverization, the composite particles are cooled to a temperature equalto or lower than an embrittlement point in a liquid nitrogen (about−196° C.) atmosphere, and then pulverization can be performed using theimpact type pulverizer. When the composite particles contain a largeamount of flexible components (for example, a silicone resin), thefreeze pulverization can cool the composite particles to a temperatureequal to or lower than the glass transition temperature (Tg) of thesilicone resin, and therefore can easily pulverize the compositeparticles.

The elastomer has a tensile breaking strength of preferably 3.0 MPa orless, more preferably 1.5 MPa or less from the above viewpoint. Theelastomer has a tensile breaking elongation of preferably 160% or less,more preferably 140% or less from the above viewpoint. The tensilebreaking strength and tensile breaking elongation of the elastomer canbe values obtained by measurement according to JIS K 6251 (2017). Alower limit value of the tensile breaking elongation is not particularlylimited, but can be 30% or more.

The elastomer has a hardness of preferably 38 or less, more preferably32 or less, as measured by a type A durometer, from the above viewpoint.The hardness of the elastomer can be a value obtained by measurementaccording to JIS K 6253-1 (2012).

The elastomer has an adhesive strength of preferably 0.3 MPa or more,more preferably 0.5 MPa or more from a viewpoint of suppressinggeneration of two peaks of the particle size by peeling of the elastomerfrom other materials (thermally conductive material and filler) inshearing during pulverization. The adhesive strength of the elastomercan be a value obtained by measurement according to JIS K 6256-1 (2013).

The elastomer preferably contains a coupling agent such as a silanecoupling agent, a titanium coupling agent, or an aluminum coupling agentfrom a viewpoint of further enhancing adhesiveness with the othermaterials to suppress generation of two peaks of the particle size bythe peeling. When the elastomer is a silicone resin (particularly roomtemperature vulcanizing (RTV) silicone resin), the coupling agentpreferably has a double bond in a molecule thereof from a viewpoint offurther enhancing the adhesiveness with the other materials.

Examples of a commercially available product of the silane couplingagent include KBM-1003, KBM-1403, KBM-502, KBM-503, KBE-1003, KBE502,KBE-503, and KBM-5103 (all of which are manufactured by Shin-EtsuChemical Co., Ltd.). Examples of a commercially available product of thetitanium coupling agent include: Plenact 55 and Plenact TTS (both ofwhich are manufactured by Ajinomoto Fine-Techno Co., Inc., “Plenact” isa registered trademark of Ajinomoto Co., Inc.); and Orgatix TC-100,Orgatix TC-401, Orgatix TC-710, and Orgatix TC-120 (all of which aremanufactured by Matsumoto Fine Chemical Co., Ltd., “Orgatix” is aregistered trademark of Matsumoto Fine Chemical Co., Ltd.). Examples ofa commercially available product of the aluminum coupling agent includePlenact AL-M (manufactured by Ajinomoto Fine-Techno Co., Inc.).

The elastomer preferably has a specific gravity of 1.1 or less from aviewpoint of increasing a difference in density between the elastomerand the filler to increase the attenuation ratio of an ultrasonic waveby the composite particles. By combining such an elastomer with tungstenoxide (specific gravity: 7.16) or thermal expansion microcapsules(specific gravity of commercial product: for example, 0.03), it iseasier to cause scattering of an ultrasonic wave at an interface betweenthe elastomer and the filler to further increase the attenuation ratioof an ultrasonic wave by the composite particles.

Examples of the filler include inorganic particles and hollow particles.Examples of the inorganic particles include ferrite, tungsten oxide,yttrium oxide, bismuth oxide, zinc oxide, zirconium oxide, tin oxide,nickel oxide, barium oxide, manganese oxide, yttrium oxide, indiumoxide, tantalum oxide, and barium titanate. Examples of the hollowparticles include glass balloons, hollow silica, cenolite, phenol resinmicroballoons, urea resin microballoons, polymethylmethacrylateballoons, and thermal expansion microcapsules. The filler may be usedsingly or in combination of two or more kinds thereof.

The composite particles can be prepared by various known methods.

The thermally conductive particles that are the composite particles havea density of preferably 1.0 to 3.5 g/cm³, more preferably 1.5 to 3.0 μm.When the density of the composite particles is within the above range,it is easy to control the acoustic impedance of the backing materialwithin a desired range even when the amount of the composite particlesadded is small.

The thermally conductive particles that are the composite particles havea number average particle size of preferably 100 to 500 μm, morepreferably 150 to 300 μm. When the number average particle size of thethermally conductive particles is within the above range, the thermallyconductive particles are more easily dispersed, and the orientationstate of the thermally conductive particles is more easily adjusted ascompared with a case of using particles having a smaller number averageparticle size.

1-5-3. Non-Thermally Conductive Particles

The backing material 170 may contain non-thermally conductive particlesin addition to the thermally conductive particles from a viewpoint ofadjusting the orientation state of the thermally conductive particles toeasily adjust the acoustic characteristics. In particular, when thebacking material 170 contains non-thermally conductive particles havinga relatively large particle size, the thermally conductive particles canbe oriented along an interface of the non-thermally conductiveparticles. As a result, a heat transfer path is formed by the thermallyconductive particles, and the thermal conductivity of the backingmaterial 170 can be efficiently enhanced even if the amount of thethermally conductive particles is small. In addition, this suppressesagglomeration of the thermally conductive particles and also suppressesa variation in the acoustic characteristics of the backing material 170.In addition, by orienting the thermally conductive particles along aninterface of the non-thermally conductive particles, it is possible tosuppress formation of an aggregate of the thermally conductiveparticles. Therefore, it is possible to suppress generation of bubblesand cracks during molding of the backing material 170, to improve thedurability of the backing material 170, and to suppress a change incapacity during dicing. Note that the non-thermally conductive particleshave a number average particle size of preferably 100 to 350 μm, morepreferably 150 to 260 μm.

The non-thermally conductive particles are preferably compositeparticles from a viewpoint of easily adjusting the acousticcharacteristics of the backing material 170.

The non-thermally conductive particles that are composite particles canhave a similar configuration to the above-described thermally conductiveparticles that are composite particles except for containing nothermally conductive material.

1-5-4. Base Material (Matrix Resin)

The backing material 170 contains a matrix resin as a base material.

The matrix resin may be a thermosetting resin such as a syntheticrubber, a natural lubber, or an epoxy resin, or may be a thermoplasticresin such as polyethylene or nylon. The matrix resin is preferably anepoxy resin or nylon.

Examples of the epoxy resin include a bisphenol type epoxy resin such asa bisphenol A type or a bisphenol F type, a novolac type epoxy resinsuch as a resole novolac type or a phenol modified novolac type, apolycyclic aromatic epoxy resin such as a naphthalenestructure-containing type, an anthracene structure-containing type, or afluorene structure-containing type, a hydrogenated alicyclic epoxyresin, and a liquid crystalline epoxy resin. The epoxy resin may be usedsingly or in combination of two or more kinds thereof. Examples of thenylon include nylon 6, nylon 11, nylon 12, and nylon 66.

The epoxy resin or nylon, which is a raw material of the matrix resin,is preferably in a form of powder particles. By using the matrix resinmade of powder particles, due to an excluded volume effect of particlessuch as the thermally conductive particles contained in the backingmaterial, the powder particles can be arranged along an interface(surface) of the thermally conductive particles. As a result, thethermally conductive particles can be oriented at regular intervals evenif the thermally conductive particles are not uniformly dispersed at thetime of mixing. Furthermore, by pressurization during molding, theorientation of the thermally conductive particles can be improved. As aresult, the thermal conductivity of the backing material can be improvedup to 30 W/mk in the thickness direction and up to 6 W/mk in thehorizontal direction.

The matrix resin has a glass transition temperature (Tg) of preferably30° C. or higher and 200° C. or lower, more preferably 50° C. or higherand 150° C. or lower, still more preferably 60° C. or higher and 100° C.or lower. When the glass transition temperature (Tg) of thethermosetting resin is 80° C. or higher, even if the temperature insidethe ultrasonic probe 100 rises to about 50° C. to 60° C. during use ofthe ultrasonic probe 100, the thermosetting resin can maintain apredetermined hardness. Therefore, deformation of the backing material170 due to softening of the base material can be suppressed. When theglass transition temperature (Tg) of the thermosetting resin is 200° C.or lower, the backing material 170 can be hardened to such an extentthat the backing material 170 is easily cut during processing of thebacking material 170, and the brittleness of the backing material 170can be reduced to such an extent that the backing material 170 is notdamaged during cutting of the backing material 170.

Note that the content of the matrix resin in the backing material 170 ispreferably 40% by volume or more and 64% by volume or less with respectto the total volume of the backing material 170.

2. Method for Preparing Backing Material

The backing material 170 can be prepared by a method including a step ofmixing a raw material of the matrix resin, the thermally conductiveresin, and a non-thermally conductive material to prepare a mixture, anda step of molding the mixture.

The mixture may contain the raw material of the matrix resin, thethermally conductive material, the non-thermally conductive material,and other additives at a ratio according to the above-describedconfiguration of the backing material 170.

Alternatively, the mixture may contain the raw material of the matrixresin, the thermally conductive resin, and the non-thermally conductivematerial.

Alternatively, the mixture may contain the raw material of the matrixresin in a form of powder and the thermally conductive resin.

The matrix resin is preferably made of powder particles having a numberaverage particle size of 10 μm or more and 200 μm or less, morepreferably powder particles having a number average particle size of 10μm or more and 100 μm or less, still more preferably powder particleshaving a number average particle size of 30 μm or more 70 μm or less. Bymaking the number average particle size of the raw material of thematrix resin smaller than that of the composite particles to be added asthe thermally conductive material, it is possible to suppress crackingduring molding, and to completely fill a gap between the particles whenthe raw material is melted.

When the raw material of the matrix resin is in a form of powder, thepowder particles of the resin can be arranged along an interface(surface) of the thermally conductive particles. Therefore, thethermally conductive particles in the backing material pressurizedduring molding can be oriented in the thickness of the backing material.In addition, the backing material can have a region where the thermallyconductive particles are aggregated and a region where the thermallyconductive particles are not aggregated. As a result, the orientation ofthe thermally conductive particles can be improved. Therefore, thethermal conductivity of the backing material can be improved to 30 W/mkin the thickness direction and 6 W/mk in the horizontal direction.

When the raw material of the matrix resin is in a form of powder, thesufficiently mixed mixture is put in a die and heated while beingpressurized in the thickness direction under vacuum deaeration, and themixture can be thereby molded into the shape of the backing material170.

When the raw material of the matrix resin is in a form of liquid, theliquid raw material is poured into a die, sufficiently defoamed,stirred, and then heated, and the mixture can be thereby molded into theshape of the backing material 170.

Note that when the raw material of the matrix resin is a thermosettingresin, a curing agent is preferably added to the mixture. Examples ofthe curing agent include: a chain aliphatic polyamine such asdiethylenetriamine, triethylenetetramine, dipropylenediamine, ordiethylaminopropylamine; a cyclic aliphatic polyamine such asN-aminoethylpiperazine, mensendiamine, or isophoronediamine; an aromaticamine such as m-xylenediamine, metaphenylenediamine,diaminodiphenylmethane, or diaminodiphenylsulfone; a polyamide resin; asecondary amine or a tertiary amine such as piperidine,N,N-dimethylpiperazine, triethylenediamine,2,4,6-tris(dimethylaminomethyl) phenol, benzyldimethylamine, or2-(dimethylaminomethyl) phenol; an imidazole such as 2-methylimidazole,2-ethylimidazole, or 1-cyanoethyl-2-undecylimidazolium trimellitate; aliquid polymercaptan and a polysulfide; and an acid anhydride such asphthalic anhydride, trimellitic anhydride, methyltetrahydrophthalicanhydride, methylendomethylenetetrahydrophthalic anhydride,methylbutenyltetrahydrophthalic anhydride, or methylhexahydrophthalicacid. The curing agent may be used singly or in combination of two ormore kinds thereof.

3. Ultrasonic Diagnostic Apparatus

FIG. 3 is a schematic view illustrating an example of the ultrasonicdiagnostic apparatus 10 including the ultrasonic probe 100. Theultrasonic diagnostic apparatus 10 includes the ultrasonic probe 100,the main body 11, a connector 12, and a display 13.

The ultrasonic probe 100 is connected to the ultrasonic diagnosticapparatus 10 via a cable 14 connected to the connector 12.

An electric signal (transmission signal) transmitted from the ultrasonicdiagnostic apparatus 10 is transmitted to the piezoelectric element 110of the ultrasonic probe 100 via the cable 14. The transmission signal isconverted into an ultrasonic wave in the piezoelectric element 110 andtransmitted into a living body. The transmitted ultrasonic wave isreflected by a tissue or the like in the living body. A part of thereflected wave is received again by the piezoelectric element 110,converted into an electric signal (reception signal), and transmitted tothe main body 11 of the ultrasonic diagnostic apparatus 10. Thereception signal is converted into image data in the main body 11 of theultrasonic diagnostic apparatus 10 and displayed on the display 13.

The ultrasonic diagnostic apparatus according to the embodiment of thepresent invention includes the ultrasonic probe according to theembodiment of the present invention, and therefore can generate anultrasonic image with favorable image quality.

EXAMPLES

Hereinafter, the present invention will be described more specificallyusing the following tests, but the present invention is not limited tothe following tests.

1. Preparation of Composite Particles

Using the following materials, non-thermoplastic particles that werecomposite particles and thermoplastic particles that were compositeparticles were prepared.

(Elastomer)

-   -   Main agent 1: TSE3032(A) (manufactured by Momentive Performance        Materials, Inc., thermosetting liquid silicone rubber)    -   Curing agent 1: TSE3032(B) (manufactured by Momentive        Performance Materials, Inc.)    -   Main agent 2: TSE3033(A) (manufactured by Momentive Performance        Materials, Inc., thermosetting liquid silicone rubber)    -   Curing agent 2: TSE3033(B) (manufactured by Momentive        Performance Materials, Inc.)

(Filler)

Filler 1: A2-WO₃ (number average particle size: 7 to 12 μm, manufacturedby A.L.M.T Corp., tungsten trioxide powder)

Filler 2: C3-WO₃ (number average particle size: 15 to 20 μm,manufactured by A.L.M.T Corp., tungsten trioxide powder)

Filler 3: Expancel 920DE40d30 (number average particle size: 35 to 55μm, manufactured by Japan Fillite Co., Ltd., thermal expansionmicrocapsules)

Note that an elastomer obtained by a reaction between main agent 1 andcuring agent 1 has

a tensile breaking strength of 4.5 MPa, measured according to JIS K 6251(2017), a tensile breaking elongation of 210%, measured according to JISK 6251 (2017), and a hardness of 35, measured by a type A durometer.

An elastomer obtained by a reaction between main agent 2 and curingagent 2 has a tensile breaking strength of 1.0 MPa, measured accordingto JIS K 6251 (2017), a tensile breaking elongation of 130%, measuredaccording to JIS K 6251 (2017), and a hardness of 30, measured by a typeA durometer.

(Particles Made of Thermally Conductive Material)

Particles 1: iGrafen-α (number average particle size: 100 μm,manufactured by ITEC Co., Ltd., multilayer graphene)

1-1. Composite Particles 1

To 100 parts by mass of main agent 1, 803 parts by mass of filler 1 wasadded, and the resulting mixture was sufficiently mixed with a vacuummixer “ARV-310” (manufactured by Thinky Corporation). Subsequently, 10parts by mass of curing agent 1 was added thereto and mixed well toobtain mixture 1.

Mixture 1 was put into a die of 100 mm×100 mm×30 mm, was allowed tostand under vacuum at room temperature for three hours at a pressure of4.9 MPa (50 kg/cm²) with a vacuum electrothermal press machine, and thenwas heated at 50° C. for three hours to prepare block 1. Block 1 had adensity of 4.07 g/cm³. Block 1 was cut into cubes each having a sidelength of 1 cm, and roughly pulverized at room temperature with a cuttermill “VM-20” (manufactured by Makino MFG. Co., Ltd.). Thereafter, theresulting product was finely pulverized with a pin mill “M-4”(manufactured by Nara Machinery Co., Ltd.) using a screen of 0.5 mm at arotation speed of 2800 rpm. Finally, the resulting product was sievedwith a circular vibrating screener “KG-400” (manufactured by NishimuraMachine Works Co., Ltd., mesh size 212 μm) to obtain composite particles1 which were non-thermally conductive particles.

Using a laser type particle size distribution measuring device (LMS-30(manufactured by Seishin Enterprise Co., Ltd.)), the particle sizedistribution of composite particles 1 was measured under stirring andultrasonic dispersion by using isopropyl alcohol as a measurementdispersion medium and adjusting an optimum point of scatteringintensity. As a result of measuring the particle sizes, the numberaverage particle size was 112 μm.

1-2. Composite Particles 2

To 50 parts by mass of main agent 2, 365 parts by mass of filler 2 and0.91 parts by mass of filler 3 were added, and the resulting mixture wassufficiently mixed with a vacuum mixer. Subsequently, 50 parts by massof curing agent 2 was added thereto and mixed well to obtain mixture 2.

Mixture 2 was put into a die of 100 mm×100 mm×30 mm, was allowed tostand under vacuum at room temperature for three hours at a pressure of4.9 MPa (50 kg/cm²) with a vacuum electrothermal press machine, and thenwas heated at 50° C. for three hours to prepare block 2. Block 2 had adensity of 2.29 g/cm³. Block 2 was cut into cubes each having a sidelength of 1 cm, and roughly pulverized at room temperature with thecutter mill. Thereafter, the resulting product was finely pulverizedwith the pin mill using a screen of 0.5 mm at a rotation speed of 2800rpm. Finally, the resulting product was sieved with the circularvibrating screener (mesh size 212 μm) to obtain composite particles 2which were non-thermally conductive particles. As a result of measuringthe number average particle size in a similar manner to the compositeparticles 1, the number average particle size was 245 μm.

1-3. Composite Particles 3

To 50 parts by mass of main agent 2, 365 parts by mass of filler 2 and1.53 parts by mass of filler 3 were added, and the resulting mixture wassufficiently mixed with a vacuum mixer. Subsequently, 50 parts by massof curing agent 2 was added thereto and mixed well to obtain mixture 3.

Mixture 3 was put into a die of 100 mm×100 mm×30 mm, was allowed tostand under vacuum at room temperature for three hours at a pressure of4.9 MPa (50 kg/cm²) with a vacuum electrothermal press machine, and thenwas heated at 50° C. for three hours to prepare block 3. Block 3 had adensity of 2.27 cm². Block 3 was cut into cubes each having a sidelength of 1 cm, and roughly pulverized at room temperature with thecutter mill. Thereafter, the resulting product was finely pulverizedwith the pin mill using a screen of 0.5 mm at a rotation speed of 2800rpm. Finally, the resulting product was sieved with the circularvibrating screener (mesh size 212 μm) to obtain composite particles 3which were thermally conductive particles. As a result of measuring thenumber average particle size in a similar manner to the compositeparticles 1, the number average particle size was 253 μm.

1-4. Composite Particles 4

To 50 parts by mass of main agent 2, 153 parts by mass of filler 2 and3.96 parts by mass of filler 3 were added, and the resulting mixture wassufficiently mixed with a vacuum mixer. Subsequently, 50 parts by massof curing agent 2 was added thereto and mixed well to obtain mixture 4.

Mixture 4 was put into a die of 100 mm×100 mm×30 mm, was allowed tostand under vacuum at room temperature for three hours at a pressure of4.9 MPa (50 kg/cm²) with a vacuum electrothermal press machine, and thenwas heated at 50° C. for three hours to prepare block 4. Block 4 had adensity of 1.40 cm³. Block 4 was cut into cubes each having a sidelength of 1 cm, and roughly pulverized at room temperature with thecutter mill. Thereafter, the resulting product was finely pulverizedwith the pin mill using a screen of 0.5 mm at a rotation speed of 2800rpm. Finally, the resulting product was sieved with the circularvibrating screener (mesh size 212 μm) to obtain composite particles 4which were thermally conductive particles. As a result of measuring thenumber average particle size in a similar manner to the compositeparticles 1, the number average particle size was 220 μm.

1-5. Composite Particles 5

To 50 parts by mass of main agent 2, 153 parts by mass of filler 2 and3.96 parts by mass of filler 3 were added, and the resulting mixture wassufficiently mixed with a vacuum mixer. Subsequently, 50 parts by massof curing agent 2 was added thereto and mixed well to obtain mixture 5.

Mixture 5 was put into a die of 100 mm×100 mm×30 mm, was allowed tostand under vacuum at room temperature for three hours at a pressure of4.9 MPa (50 kg/cm²) with a vacuum electrothermal press machine, and thenwas heated at 50° C. for three hours to prepare block 5. Block 5 had adensity of 1.02 cm³. Block 5 was cut into cubes each having a sidelength of 1 cm, freeze-pulverized, and finally sieved with the circularvibrating screener (mesh size 212 μm) to obtain composite particles 5which were thermally conductive particles. As a result of measuring thenumber average particle size in a similar manner to the compositeparticles 1, the number average particle size was 254 μm.

In the freeze pulverization, the composite particles are cooled to atemperature equal to or lower than an embrittlement point in a liquidnitrogen (about −196° C.) atmosphere, and then pulverization can beperformed using an impact type pulverizer such as a turbo mill, a pinmill, a hammer mill, or a Linrex mill.

1-5. Composite Particles 6

To 100 parts by mass of main agent 2, 175 parts by mass of filler 2 and1.30 parts by mass of filler 3 were added, and the resulting mixture wassufficiently mixed with a vacuum mixer. Subsequently, 10 parts by massof curing agent 2 was added thereto and mixed well to obtain mixture 6.

Mixture 6 was put into a die of 100 mm×100 mm×30 mm, was allowed tostand under vacuum at room temperature for three hours at a pressure of4.9 MPa (50 kg/cm²) with a vacuum electrothermal press machine, and thenwas heated at 50° C. for three hours to prepare block 6. Block 6 had adensity of 1.69 cm³. Block 6 was cut into cubes each having a sidelength of 1 cm, and roughly pulverized at room temperature with thecutter mill. Thereafter, the resulting product was finely pulverizedwith the pin mill using a screen of 0.5 mm at a rotation speed of 2800rpm. Finally, the resulting product was sieved with the circularvibrating screener (mesh size 212 μm) to obtain composite particles 6which were thermally conductive particles. As a result of measuring thenumber average particle size in a similar manner to the compositeparticles 1, the number average particle size was 233 μm.

1-5. Composite Particles 7

To 50 parts by mass of main agent 2, 365 parts by mass of filler 2, 1.53parts by mass of filler 3, and 50 parts by mass of particles 1 wereadded, and the resulting mixture was sufficiently mixed with a vacuummixer. Subsequently, 50 parts by mass of curing agent 2 was addedthereto and mixed well to obtain mixture 7.

Mixture 7 was put into a die of 100 mm×100 mm×30 mm, was allowed tostand under vacuum at room temperature for three hours at a pressure of4.9 MPa (50 kg/cm²) with a vacuum electrothermal press machine, and thenwas heated at 50° C. for three hours to prepare block 7. Block 7 had adensity of 2.35 cm³. Block 7 was cut into cubes each having a sidelength of 1 cm, and roughly pulverized at room temperature with thecutter mill. Thereafter, the resulting product was finely pulverizedwith the pin mill using a screen of 0.5 mm at a rotation speed of 2800rpm. Finally, the resulting product was sieved with the circularvibrating screener (mesh size 212 μm) to obtain composite particles 7which were thermally conductive particles. As a result of measuring thenumber average particle size in a similar manner to the compositeparticles 1, the number average particle size was 252 μm.

Table 1 illustrates the content, density, and number average particlesize of each of components forming composite particles 1 to 7 describedabove.

TABLE 1 Elastomer Thermally Number Main agent Curing agent Fillerconductive average Parts Parts Parts Parts particle particle by by by byParts by Density size Kind mass Kind mass Kind mass Kind mass Kind massg/cm³ μm Composite Main agent 1 100 Curing agent 1 10 Filler 1 803 — — —— 4.07 112 particle 1 Composite Main agent 2 50 Curing agent 2 50 Filler2 365 Filler 3 0.91 — — 2.29 245 particle 2 Composite Main agent 2 50Curing agent 2 50 Filler 2 365 Filler 3 1.53 — — 2.27 253 particle 3Composite Main agent 2 50 Curing agent 2 50 Filler 2 153 Filler 3 3.96 —— 1.40 220 particle 4 Composite Main agent 2 50 Curing agent 2 50 Filler2 153 Filler 3 3.96 — — 1.02 254 particle 5 Composite Main agent 2 100Curing agent 2 10 Filler 2 175 Filler 3 1.30 — — 1.69 233 particle 6Composite Main agent 2 50 Curing agent 2 50 Filler 2 365 Filler 3 1.53Particle 1 50 2.35 252 particle 7

2. Preparation of Backing Material

A backing material was prepared using the following materials.

(Raw Material of Matrix Resin)

Main agent 3: Albidur EP2240 (manufactured by Evonik Industries AG,liquid epoxy resin)

Curing agent 3: jER Cure ST-12 (manufactured by Mitsubishi ChemicalCorporation)

Main agent 4: jER 828 (manufactured by Mitsubishi Chemical Corporation,liquid epoxy resin)

Curing agent 4: jER Cure 113 (manufactured by Mitsubishi ChemicalCorporation)

Main agent 5: PCE-751 (number average particle size: 45 to 50 μm,manufactured by Pelnox Limited, powder epoxy resin)

Main agent 6: F-246 (number average particle size: 45 μm, manufacturedby Somar Corporation, powder epoxy resin)

Main agent 7: Rilsan fine powder (number average particle size: 30 μm,manufactured by Arkema K.K., powder nylon 11 resin)

(Non-Thermally Conductive Particles)

Composite particles 1: Composite particles 1 prepared above

Composite particles 3: Composite particles 3 prepared above

Composite particles 5: Composite particles 5 prepared above

(Thermally Conductive Particles)

Particles 1: iGrafen-α (manufactured by ITEC Co., Ltd., multilayergraphene)

Composite particles 7: Composite particles 7 prepared above

Note that the thermally conductive material (multilayer graphene)forming particles 1 and composite particles 7 each have a thermalconductivity of 1300 W/mk.

Particles 1 had a density of 2.2 g/cm³, and composite particles 7 had adensity of 2.35 g/cm³.

2-1. Backing Material 1

76.0 parts by mass of main agent 3 and 730 parts by mass of compositeparticles 1 were sufficiently mixed with a vacuum mixer. 24.0 parts bymass of curing agent 3 was further added thereto, and further mixed toobtain a compound.

The compound was put into a die of 100 mm×100 mm×30 mm, and allowed tostand at room temperature for four hours while being pressurized at apressure of 9.9 MPa (100 kg/cm²) with a vacuum electrothermal pressmachine “OHV-H” (manufactured by Oji Machine Co., Ltd.). Thereafter, theresulting product was heated at 80° C. for three hours to obtain backingmaterial 1.

2-2. Backing Material 2

76.0 parts by mass of main agent 3 and 97.0 parts by mass of particles 1were sufficiently mixed with a vacuum mixer. 24.0 parts by mass ofcuring agent 3 was further added thereto, and further mixed to obtain acompound.

The compound was put into a die of 100 mm×100 mm×30 mm, and allowed tostand at room temperature for four hours while being pressurized at apressure of 9.9 MPa (100 kg/cm²) with the vacuum electrothermal pressmachine. Thereafter, the resulting product was heated at 80° C. forthree hours to obtain backing material 2.

2-3. Backing Material 3

76.0 parts by mass of main agent 4, 160.0 parts by mass of compositeparticles 3, and 13.8 parts by mass of particles 1 were sufficientlymixed with a vacuum mixer. 24.0 parts by mass of curing agent 4 wasfurther added thereto, and further mixed to obtain a compound.

The compound was put into a die of 100 mm×100 mm×30 mm, and allowed tostand at room temperature for four hours while being pressurized at apressure of 9.9 MPa (100 kg/cm²) with the vacuum electrothermal pressmachine. Thereafter, the resulting product was heated at 80° C. forthree hours to obtain backing material 3.

2-4. Backing Material 4

75.8 parts by mass of main agent 4, 160.0 parts by mass of compositeparticles 3, and 35.0 parts by mass of particles 1 were sufficientlymixed with a vacuum mixer. 24.2 parts by mass of curing agent 4 wasfurther added thereto, and further mixed to obtain a compound.

The compound was put into a die of 100 mm×100 mm×30 mm, and allowed tostand at 80° C. for one hour while being pressurized at a pressure of9.9 MPa (100 kg/cm²) with the vacuum electrothermal press machine.Thereafter, the resulting product was heated at 150° C. for three hoursto obtain backing material 4.

2-5. Backing Material 5

Into a rocking mixer RM-10 (manufactured by Aichi Electric Co., Ltd.),100 parts by mass of main agent 5, 160.0 parts by mass of compositeparticles 3, and 35.0 parts by mass of particles 1 were put, and mixedfor 20 minutes at a rotation speed of 19 rpm at a rocking speed of 11rpm to obtain a powder mixture.

The powder mixture was put into a die of φ200 mm×100 mm, and heated at150° C. for four hours while being pressurized at a pressure of 9.9 MPa(100 kg/cm²) with the vacuum electrothermal press machine to obtainbacking material 5.

2-6. Backing Material 6

A backing material 6 was obtained in a similar manner to the backingmaterial 5 except that the amount of particles 1 added was changed from35.0 parts by mass to 53.0 parts by mass.

2-7. Backing Material 7

Into a rocking mixer RM-10 (manufactured by Aichi Electric Co., Ltd.),100 parts by mass of main agent 5, 73.0 parts by mass of compositeparticles 5, and 35.0 parts by mass of particles 1 were put, and mixedfor 20 minutes at a rotation speed of 19 rpm at a rocking speed of 11rpm to obtain a powder mixture.

The powder mixture was put into a die of φ200 mm×100 mm, and heated at150° C. for four hours while being pressurized at a pressure of 9.9 MPa(100 kg/cm²) with the vacuum electrothermal press machine to obtainbacking material 7.

2-8. Backing Material 8

Into a rocking mixer RM-10 (manufactured by Aichi Electric Co., Ltd.),100 parts by mass of main agent 6, 160.0 parts by mass of compositeparticles 3, and 53.0 parts by mass of particles 1 were put, and mixedfor 20 minutes at a rotation speed of 19 rpm at a rocking speed of 11rpm to obtain a powder mixture.

The powder mixture was put into a die of φ200 mm×100 mm, and heated at150° C. for four hours while being pressurized at a pressure of 9.9 MPa(100 kg/cm2) with the vacuum electrothermal press machine to obtainbacking material 8.

2-9. Backing Material 9

Into a rocking mixer RM-10 (manufactured by Aichi Electric Co., Ltd.),100 parts by mass of main agent 7, 160.0 parts by mass of compositeparticles 3, and 53.0 parts by mass of particles 1 were put, and mixedfor 20 minutes at a rotation speed of 19 rpm at a rocking speed of 11rpm to obtain a powder mixture.

The powder mixture was put into a die of φ200 mm×100 mm, and heated at150° C. for four hours while being pressurized at a pressure of 9.9 MPa(100 kg/cm2) with the vacuum electrothermal press machine to obtainbacking material 9.

2-10. Backing Material 10

Into a rocking mixer RM-10 (manufactured by Aichi Electric Co., Ltd.),100 parts by mass of main agent 5, 160.0 parts by mass of compositeparticles 7, and 53.0 parts by mass of particles 1 were put, and mixedfor 20 minutes at a rotation speed of 19 rpm at a rocking speed of 11rpm to obtain a powder mixture.

The powder mixture was put into a die of φ200 mm×100 mm, and heated at150° C. for four hours while being pressurized at a pressure of 9.9 MPa(100 kg/cm2) with the vacuum electrothermal press machine to obtainbacking material 10.

Table 2 illustrates the content and glass transition temperature (Tg) ofeach of components forming backing materials 1 to 10.

TABLE 2 Non-thermally Thermally Thermally conductive Base agentconductive particle conductive particle Main agent Curing agent(Composite particle) particle (Composite particle) Parts Parts PartsParts Parts by by by by by Tg Backing Kind mass Kind mass Kind mass Kindmass Kind mass (° C.) Note 1 Main agent 3 76.0 Curing 24.0 Compositeparticle 1 730 — — — — 67 Comparative agent 3 Example 2 Main agent 376.0 Curing 24.0 — — Particle 1 97.0 — — 72 Comparative agent 3 Example3 Main agent 4 76.0 Curing 24.0 Composite particle 3 160.0 Particle 113.8 — — 118 Comparative agent 4 Example 4 Main agent 4 75.8 Curing 24.2Composite particle 3 160.0 Particle 1 35.0 — — 117 Comparative agent 4Example 5 Main agent 5 100.0 — — Composite particle 3 160.0 Particle 135.0 — — 158 Example 6 Main agent 5 100.0 — — Composite particle 3 160.0Particle 1 53.0 — — 155 Example 7 Main agent 5 100.0 — — Compositeparticle 5 73.0 Particle 1 35.0 — — 164 Example 8 Main agent 6 100.0 — —Composite particle 3 160.0 Particle 1 53.0 — — 109 Example 9 Main agent7 100.0 — — Composite particle 3 160.0 Particle 1 53.0 — — 33 Example 10Main agent 5 100.0 — — — — Particle 1 53.0 Composite 160.0 162 Exampleparticle 7

3. Physical Properties of Backing Material

The physical properties (acoustic impedance, attenuation ratio, thermalconductivity, and sound velocity) of each of the backing materials 1 to10 were measured by the following methods.

3-1. Acoustic Impedance

The acoustic impedance was determined according to JIS Z2353:2003.Specifically, the sound velocity was measured using a sing-around soundvelocity measuring device (manufactured by Ultrasonic Engineering Co.,Ltd.) at 25° C., and the acoustic impedance was calculated according tothe following formula (1).

acoustic impedance (Z: MRayls)=density (ρ:×10³ kg/m³)×sound velocity(C:×10³ msec)  Formula (1):

3-2. Attenuation Ratio

The attenuation ratio of an ultrasonic wave was determined according toJIS Z2354:2012. Specifically, a water tank was filled with water at 25°C. Using an ultrasonic pulser/receiver “JPR-10C” (manufactured by JapanProbe Co., Ltd.), an ultrasonic wave of 1 MHz was generated in water,and the magnitude of an amplitude was measured before and after theultrasonic wave passed through the sheet.

3-3. Thermal Conductivity

The thermal conductivity was determined by a laser flash methodaccording to JIS R1611:2010.

Specifically, the thermal conductivity of each of backing materials 1 to10 (size of test piece: φ10 mm, t=2 mm) was measured with LFA-502(manufactured by Kyoto Electronics Manufacturing Co., Ltd.).

3-4. Sound Velocity

The sound velocity was calculated according to the formula (1) using theabove-described value of the acoustic impedance (Z: Mrayls) determinedaccording to JIS Z2353:2003.

Table 3 illustrates the physical properties of each of backing materials1 to 10.

TABLE 3 Physical properties Thermal conductivity (W/mk) Sound velocity(m/s) Attenuation Thickness Thickness Acoustic ratio direction/direction/ impedance (dB/min · Thickness Horizontal Horizontal ThicknessHorizontal Horizontal Backing (MRay1s) MHz) direction directiondirection direction direction direction Note 1 2.76 8.4 0.5 0.5 1.001043 1039 1.00 Comparative Example 2 4.06 3.6 6.3 6.4 0.98 1950 19700.99 Comparative Example 3 2.60 6.0 3.1 3.0 1.03 1433 1440 1.00Comparative Example 4 2.57 7.8 4.3 4.5 0.96 1431 1424 1.00 ComparativeExample 5 2.54 11.0 25.0 4.6 5.43 1201 1895 0.63 Example 6 2.69 15.029.9 6.5 4.60 1185 1789 0.66 Example 7 1.75 13.5 26.5 5.2 5.10 1385 20050.69 Example 8 2.64 11.0 29.9 6.5 4.60 1245 1846 0.67 Example 9 2.6011.0 25.4 4.8 5.30 1220 1806 0.67 Example 10 2.71 9.7 30.2 6.8 4.44 11531735 0.66 Example

It was found that backing materials 5 to 10 each containing a powderbase material exhibited higher attenuation ratios than backing materials1 to 4 each containing a liquid epoxy resin. It is considered that thisis because the orientation of the particles could be easily controlledby forming the thermally conductive particles into composite particles.This makes it possible to obtain a high attenuation ratio even when thecontent of the thermally conductive particles is reduced. Here, thelarger the attenuation ratio of the backing material is, the less thereflection of an ultrasonic wave from a back surface side of apiezoelectric body is. Therefore, deterioration of a diagnostic imagecan be suppressed.

By changing the matrix resin used from a liquid epoxy resin to a powderepoxy resin, it was possible to obtain a backing material having a highattenuation ratio and high thermally conductive performance asillustrated in backing materials 5 to 10. Backing materials 5 to 10 canefficiently dissipate heat generated by a piezoelectric element due to ahigh thermal conductivity, and therefore can suppress overheat of anacoustic lens in contact with a subject. In addition, backing materials5 to 10 each have a high attenuation ratio of an ultrasonic wave, andtherefore can suppress the reflection of an ultrasonic wave transmittedto a back surface side. This makes it possible to obtain a high-qualitytomographic image.

FIG. 4A is a graph illustrating heat dissipation effects of backingmaterials 1, 3 and 5 when an input voltage is 60 Vpp. FIG. 4B is a graphillustrating heat dissipation effects of backing materials 1, 3 and 5when the input voltage is 100 Vpp. Note that the rising temperature ofthe acoustic lens illustrated in FIGS. 4A and 4B indicates a valueobtained by measurement using a thermography “FLIRC2” (manufactured byFLIR Systems, Inc.).

It is found from FIGS. 4A and 4B that use of the backing materialaccording to the embodiment of the present invention reduced heatgeneration of the acoustic lens even when a high voltage was applied. Itis considered that this is because the amount of the thermallyconductive particles used can be reduced by forming the thermallyconductive particles into composite particles.

(Processability of Backing Material)

Backing materials 1 to 10 were evaluated for moldability, durability,and dicing property.

(Moldability)

Evaluation was performed using backing materials 1 to 10 molded into adiameter of 50 mm and a height of 20 mm by the above-described method.

(Evaluation Method)

Backing materials 1 to 10 were cut with a wire saw “CS-203”(manufactured by Musashino Denshi, Inc.) and further polished to athickness of 10 mm with a precision polishing device “MA-200”(manufactured by Musashino Denshi, Inc.). The resulting backingmaterials 1 to 10 were observed with an optical microscope and visually,and bubbles and cracks thereof were checked. Note that evaluationcriteria of A and B were acceptable.

(Evaluation Criteria)

A: No bubble or crack is generated, and no uneven distribution ofparticles is observed

B: The number of bubbles and cracks is less than 3, and no unevendistribution of particles is observed

C: The number of bubbles and cracks is less than 6, and unevendistribution of some particles is observed

D: The number of bubbles and cracks is 6 or more, and unevendistribution of particles is observed

(Durability)

Each of backing materials 1 to 10 prepared by the above-described methodwas cut into a size of 30 mm×30 mm×1 mm. Using this product as a testpiece, the test piece was immersed in oleic acid at 50° C., and aswelling condition thereof was checked. Note that evaluation criteria ofA and B were acceptable.

(Evaluation Criteria)

A: Swelling degree is less than 3%

B: Swelling degree is 3% or more and less than 5%

C: Swelling degree is 5% or more and less than 10%

D: Swelling degree is 10% or more

(Dicing Property)

A matching layer, a piezoelectric material, a flexible printed circuitboard (FPC), a backing material, and the like are bonded to each otherinto a TD shape. By dicing the resulting product with a 20 μm blade at a50 μm pitch with an aspect ratio of about 6 (total 300 μm filmthickness), 500 pieces were prepared. Among the 500 pieces, the numberof pieces whose capacity had been changed from a theoretical value waschecked. Note that evaluation criteria of A, B, and C were acceptable.

(Evaluation Criteria)

A: less than 3/500

B: less than 10/500

C: less than 200/500

D: 200/500 or more

Table 4 illustrates the moldability, durability, and dicing property ofeach of backing materials 1-10.

TABLE 4 Processability Backing Moldability Durability Dicing propertyNote 1 A C A Comparative Example 2 D D C Comparative Example 3 A A BComparative Example 4 C B A Comparative Example 5 A A A Example 6 A A AExample 7 B A A Example 8 A A A Example 9 A B A Example 10 A A A Example

By using a powder resin as a raw material of the matrix resin, it waspossible to obtain a backing material having excellent moldability,durability, and dicing property. It is considered that this is becauseby using powder particles as a raw material of the matrix resin, due toan excluded volume effect of particles such as the thermally conductiveparticles contained in the backing material, the thermally conductiveparticles can be oriented at regular intervals even if the thermallyconductive particles are not uniformly dispersed at the time of mixing.Furthermore, it is considered that this is because by pressurizationduring molding, the orientation of the thermally conductive particlescan be improved.

The ultrasonic probe according to the embodiment of the presentinvention has high heat dissipation by the backing material and littledeterioration in image quality due to increased thermally conductiveperformance of the backing material, therefore makes it possible toperform imaging at a higher voltage, and is useful as an ultrasonicprobe of an ultrasonic diagnostic apparatus having higher sensitivity.

Although embodiments of the present invention have been described andillustrated in detail, the disclosed embodiments are made for purposesof illustration and example only and not limitation. The scope of thepresent invention should be interpreted by terms of the appended claims

What is claimed is:
 1. An ultrasonic probe comprising: a piezoelectricelement; and a backing material including a matrix resin and thermallyconductive particles, arranged on one direction side with respect to thepiezoelectric element, wherein a ratio of thermal conductivity of thebacking material in a thickness direction to the thermal conductivity ofthe backing material in a horizontal direction is 3 or more.
 2. Theultrasonic probe according to claim 1, wherein the backing material hasa region where the thermally conductive particles are aggregated and aregion where the thermally conductive particles are not aggregated, andthe thermally conductive particles are oriented in the thicknessdirection of the backing material.
 3. The ultrasonic probe according toclaim 1, wherein a ratio of sound velocity of the backing material inthe thickness direction to the sound velocity of the backing material inthe horizontal direction is 0.5 or more.
 4. The ultrasonic probeaccording to claim 1, wherein the thermal conductivity of the backingmaterial in the thickness direction is 2.0 W/mk or more, and the thermalconductivity of the backing material in the horizontal direction is 2.0W/mk or more.
 5. The ultrasonic probe according to claim 1, wherein thematrix resin has a glass transition temperature (Tg) of 30° C. or higherand 200° C. or lower.
 6. The ultrasonic probe according to claim 1,wherein the backing material contains non-thermally conductiveparticles.
 7. The ultrasonic probe according to claim 6, wherein thenon-thermally conductive particles are composite particles.
 8. Anultrasonic diagnostic apparatus comprising the ultrasonic probeaccording to claim
 1. 9. A method for manufacturing a backing materialused for an ultrasonic probe, the method comprising: mixing a rawmaterial of a matrix resin, thermally conductive particles, andnon-thermally conductive particles to prepare a mixture; and molding themixture while the mixture is pressurized and heated to form a backingmaterial, wherein the thermally conductive particles are oriented in athickness direction of the backing material.
 10. The method formanufacturing a backing material according to claim 9, wherein a ratioof thermal conductivity of the backing material in the thicknessdirection to the thermal conductivity of the backing material in ahorizontal direction is 3 or more.
 11. The method for manufacturing abacking material according to claim 9, wherein a ratio of sound velocityof the backing material in the thickness direction to the sound velocityof the backing material in a horizontal direction is 0.5 or more. 12.The method for manufacturing a backing material according to claim 9,wherein the raw material of the matrix resin contains powder particles.13. The method for manufacturing a backing material according to claim9, wherein the raw material of the matrix resin contains powderparticles having a number average particle size of 10 μm or more and 200μm or less.